Method and system for detecting fault of rear oxygen sensor

ABSTRACT

A method for detecting a fault of a rear oxygen sensor installed on an exhaust pipe of a vehicle may include determining whether the rear oxygen sensor satisfies a diagnosis entry condition, comparing an output voltage of the rear oxygen sensor to a preset fault determination voltage range so as to determine whether the output voltage of the rear oxygen sensor is included in the fault determination voltage range, changing the duty of a heater of the rear oxygen sensor when the output voltage of the rear oxygen sensor is not included in the fault determination voltage range, determining whether the output voltage of the rear oxygen sensor, controlled by changing the heater duty, is included in the fault determination voltage range, and confirming that the rear oxygen sensor has a fault, when the output voltage of the rear oxygen sensor is included in the fault determination voltage range.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to Korean Patent Application No. 10-2014-0099170, filed on Aug. 1, 2014, the entire disclosure of which is hereby incorporated by reference herein.

BACKGROUND

1. Field of the Disclosure

The present disclosure relates to a method and system for detecting a fault of a rear oxygen sensor which is installed on an exhaust pipe of a vehicle; and, particularly, to a method and system for detecting a fault of a rear oxygen sensor which monitors an output voltage of the rear oxygen sensor in a state where power of a heater to activate the rear oxygen sensor is cut off for a predetermined time, and detect a fault of the rear oxygen sensor.

2. Description of Related Art

In general, a vehicle measures the concentration of oxygen contained in exhaust gas discharged from an engine using an oxygen sensor, and determines the state of the exhaust gas. Then, the vehicle controls combustion to reduce exhaust gas.

For example, as illustrated in FIG. 1, a catalytic converter 13 is installed on an exhaust pipe 12 for discharging exhaust gas from an engine 11 to the outside of the vehicle. A front oxygen sensor 14 and a rear oxygen sensor 15 are installed at the front and rear of the catalytic converter 13 on the exhaust pipe 12, and oxygen concentrations measured through the front oxygen sensor 14 and the rear oxygen sensor 15 are outputted to an electronic control unit (ECU) 16 to monitor the oxygen content of the exhaust gas at the front and rear of the catalytic converter 13. In particular, the ECU 16 monitors the oxygen storage ability of the catalytic converter 13 using the oxygen concentration measured through the rear oxygen sensor 15, and reduces exhaust gas which is finally discharged from the vehicle.

Thus, when an abnormality occurs in the rear oxygen sensor 15, the ECU 16 may not normally monitor the catalytic function of the catalytic converter 13. In this case, since the ECU 16 cannot determine whether the catalytic converter 13 has a fault, the exhaust gas may not be normally controlled.

Conventionally, a load pulse method or a voltage bias method has been used to detect a fault of the rear oxygen sensor. According to the load pulse method illustrated in FIG. 2, a voltage of +5V is forcibly outputted to an output terminal of the rear oxygen sensor 15, and compared to a preset value. According to the voltage bias method illustrated in FIGS. 3 to 4, when the rear oxygen sensor 15 is activated after startup, a voltage difference between a high input terminal 15 a and a low input terminal 15 b of the rear oxygen sensor 15 is continuously monitored to detect a fault.

The load pulse method compares voltages before and after a pulse, and determines that the rear oxygen sensor is normal when a voltage difference (A-B) is larger than a threshold value. Furthermore, the load pulse method determines that the rear oxygen sensor is shorted when the voltage difference is smaller than the threshold value, and determines that the rear oxygen sensor is opened when the voltage difference is close to a reference value.

However, the load pulse method has a problem in that the load pulse method cannot continuously monitor a fault of the rear oxygen sensor and cannot consider the state of the engine. Thus, the load pulse method cannot deal with various regulations in which the state of the engine must be considered.

Furthermore, the voltage bias method has a problem in that the voltage bias method may determine whether the rear oxygen sensor is opened according to a driver's driving habit, even though the voltage bias method can continuously monitor the rear oxygen sensor when the rear oxygen sensor is activated. In the voltage bias method, after the temperature of exhaust gas increases to a predetermined temperature, the reference voltage for the two input terminals 15 a and 15 b of the rear oxygen sensor is biased through a plurality of resistors R1, R2, and R3. Then, when the voltage difference between the high input terminal 15 a and the low input terminal 15 b, inputted to the controller, is maintained around the reference voltage for a predetermined time, the voltage bias method may detect that the rear oxygen sensor is opened. That is, as illustrated in FIG. 4, when a predetermined time elapses in a state where the voltage difference between the high and low input terminals deviates from a preset range R, the voltage bias method may determine that the rear oxygen sensor 15 is opened.

However, even when a driver operates the vehicle without stepping on a brake pedal or accelerator pedal on a downhill road, a voltage behavior may occur as illustrated in FIG. 4, showing that the state in which the rear oxygen sensor 15 has a voltage difference corresponding to the reference voltage is continued. In this case, the voltage bias method may determine that the rear oxygen sensor is opened.

SUMMARY OF THE DISCLOSURE

An embodiment of the present disclosure is directed to a method and system for detecting a fault of a rear oxygen sensor, which is capable of detecting whether the rear oxygen sensor is opened, regardless of a driver's driving habit, after the rear oxygen sensor is activated, based on the principle that the voltage of the rear oxygen sensor is changed when the heater duty of the rear oxygen sensor is changed.

Other objects and advantages of the present disclosure can be understood by the following description, and become apparent with reference to the embodiments of the present disclosure. Also, it is obvious to those skilled in the art to which the present disclosure pertains that the objects and advantages of the present disclosure can be realized by the means as claimed and combinations thereof.

In accordance with an embodiment of the present disclosure, there is provided a method for detecting a fault of a rear oxygen sensor which is installed at the rear of a catalytic converter on an exhaust pipe of a vehicle and measures the concentration of oxygen included in exhaust gas passing through the catalytic converter. The method may include: determining whether the rear oxygen sensor satisfies a diagnosis entry condition; comparing an output voltage of the rear oxygen sensor to a preset fault determination voltage range so as to determine whether the output voltage of the rear oxygen sensor is included in the fault determination voltage range; changing the duty of a heater of the rear oxygen sensor when the output voltage of the rear oxygen sensor is included in the fault determination voltage range; determining whether the output voltage of the rear oxygen sensor, controlled by changing the heater duty, is included in the fault determination voltage range; and confirming that the rear oxygen sensor has a fault, when the output voltage of the rear oxygen sensor is included in the fault determination voltage range.

In the determining whether the rear oxygen sensor satisfies the diagnosis entry condition, the diagnosis entry condition may be satisfied when the temperature of the rear oxygen sensor exceeds a preset temperature.

In the determining whether the rear oxygen sensor satisfies the diagnosis entry condition, the diagnosis entry condition may be satisfied when an engine is completely warmed up.

In determining whether the rear oxygen sensor satisfies the diagnosis entry condition, the diagnosis entry condition may be satisfied when control for an air/fuel ratio of an engine is performed.

In the determining whether the rear oxygen sensor satisfies the diagnosis entry condition, the diagnosis entry condition may be satisfied when the vehicle is not coasting.

In the step of comparing the output voltage of the rear oxygen sensor to the preset fault determination voltage range so as to determine whether the output voltage of the rear oxygen sensor is included in the fault determination voltage range, the procedure may return to the step of determining whether the rear oxygen sensor satisfies the diagnosis entry condition when the output voltage of the rear oxygen sensor is included in the fault determination voltage range.

In the step of changing the duty of the heater of the rear oxygen sensor, the duty of the heater may be changed to a minimum from a maximum when the output voltage of the rear oxygen sensor is included in the fault determination voltage range.

Changing the duty of the heater of the rear oxygen sensor may be performed while the rear oxygen sensor maintains an activation temperature.

In the step of changing the duty of the heater of the rear oxygen sensor, the duty of the heater may be changed for 4 seconds to 6 seconds when the output voltage of the rear oxygen sensor is included in the fault determination voltage range.

The method may further include, between the steps of comparing the output voltage of the rear oxygen and the changing the duty of the heater of the rear oxygen sensor when the output voltage of the rear oxygen sensor is included in the fault determination voltage range: determining that the rear oxygen sensor has a fault, when the output voltage of the rear oxygen sensor is included in the fault determination voltage range, and increasing a fault diagnosis count; and comparing the fault diagnosis count to a preset heater duty change count so as to determine whether the fault diagnosis count is larger than the heater duty change count.

In the determining whether the output voltage of the rear oxygen sensor, controlled by changing the heater duty, is included in the fault determination voltage range, when the output voltage of the rear oxygen sensor is not included in the fault determination voltage range, the fault diagnosis count may be reset, and the procedure may return to the determining whether the rear oxygen sensor satisfies the diagnosis entry condition.

In the comparing the output voltage of the rear oxygen sensor to the preset fault determination voltage range so as to determine whether the output voltage of the rear oxygen sensor is included in the fault determination voltage range and the determining whether the output voltage of the rear oxygen sensor, controlled by changing the heater duty, is included in the fault determination voltage range, the fault determination voltage ranges may be set to the same range.

In accordance with another embodiment of the present disclosure, a system for detecting a fault of a rear oxygen sensor may include: a voltage comparator configured to determine whether an output voltage of a rear oxygen sensor is included in a preset fault determination voltage range; a counter configured to increase a fault diagnosis count, when the voltage comparator determines that the output voltage of the rear oxygen sensor is included in the fault determination voltage range, and to determine whether the fault diagnosis count reaches a previously stored heater duty change count; a heater power controller configured to control power applied to a heater installed at the rear oxygen sensor such that the duty of the power is decreased when the fault diagnosis count reaches the heater duty change count; and a controller configured to (i) determine whether a condition for diagnosing a fault of the rear oxygen sensor is satisfied, based on information inputted from an electronic control unit (ECU) of a vehicle, (ii) control the counter to increase the fault diagnosis count when the voltage comparator determines that the output voltage of the rear oxygen sensor is included in the fault determination voltage range, and (iii) operate the heater power controller to decrease the duty of the heater when the fault diagnosis count stored in the counter reaches the preset heater duty change count.

The heater power controller may decrease the duty of the power applied to the heater to a minimum for a preset time.

The heater power controller may decrease the duty of the power applied to the heater for a time in which the rear oxygen sensor maintains an activation temperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a conceptual view illustrating a state in which oxygen sensors are installed at the front and rear of a catalytic converter in a typical vehicle.

FIG. 2 is a conceptual view for explaining the principle of a conventional method for detecting a fault of a rear oxygen sensor.

FIG. 3 is a conceptual view for explaining the principle of another conventional method for detecting a fault of a rear oxygen sensor.

FIG. 4 is a graph illustrating voltage changes of the rear oxygen sensor in FIG. 3.

FIG. 5 is a flowchart illustrating a method for detecting a fault of a rear oxygen sensor in accordance with an embodiment of the present disclosure.

FIG. 6 is a graph illustrating voltage changes of the rear oxygen sensor in accordance with an embodiment of the present disclosure.

FIG. 7 is a block diagram illustrating a system for detecting a fault of a rear oxygen sensor in accordance with an embodiment of the present disclosure.

DESCRIPTION OF SPECIFIC EMBODIMENTS

Exemplary embodiments of the present disclosure will be described below in more detail with reference to the accompanying drawings. The present disclosure may, however, be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the present disclosure to those skilled in the art. Throughout the disclosure, like reference numerals refer to like parts throughout the various figures and embodiments of the present disclosure.

Hereafter, a method for detecting a fault of a rear oxygen sensor in accordance with an embodiment of the present disclosure will be described in detail with reference to the accompanying drawings.

An embodiment of the present disclosure provides a method for detecting a fault of a rear oxygen sensor, which is installed at the rear of a catalytic converter on an exhaust pipe of a vehicle and measures the concentration of oxygen contained in exhaust gas passing through the catalytic converter. The method for detecting a fault of a rear oxygen sensor may include: determining whether the rear oxygen sensor 15 satisfies a diagnosis entry condition (step S110); comparing an output voltage of the rear oxygen sensor 15 to a preset fault determination voltage range R so as to determine whether the output voltage of the rear oxygen sensor 15 is included in the fault determination voltage range R (step S120); changing the duty of a heater 15 c of the rear oxygen sensor 15 when the output voltage of the rear oxygen sensor is included in the fault determination voltage range R (step S150); determining whether the output voltage of the rear oxygen sensor 15, controlled by changing the heater duty, is included in the fault determination voltage range R (step S160); and confirming that the rear oxygen sensor 15 has a fault, when it is determined at step S160 that the output voltage of the rear oxygen sensor 15 is included in the fault determination voltage range R (step S170).

At step S110, the method may determine whether the state of the rear oxygen sensor 15 installed at the rear of the catalytic converter 13 on the exhaust pipe 12 of the vehicle satisfies a diagnosis entry condition for diagnosing whether the rear oxygen sensor 15 has a fault. The diagnosis entry condition for diagnosing whether the rear oxygen sensor 15 has a fault may include the temperature of the rear oxygen sensor 15, whether the engine was warmed up, whether to control an air-fuel ratio, and whether the vehicle is coasting.

In order to diagnose whether the rear oxygen sensor 15 has a fault, the rear oxygen sensor 15 needs to be activated in such a range that the rear oxygen sensor 15 can be normally operated. Thus, the rear oxygen sensor 15 needs to satisfy the diagnosis entry condition. The method may check whether the rear oxygen sensor 15 exceeds a preset activation temperature, whether the engine 11 was warmed up, whether to control the air/fuel ratio, and whether the vehicle is coasting. When all the above-described conditions are satisfied or some of the conditions are satisfied, it may be considered that the diagnosis entry condition is satisfied. Furthermore, required conditions may be added in addition to the above-described conditions.

Thus, when the rear oxygen sensor 15 satisfies the diagnosis entry condition, the following processes may be sequentially performed. Otherwise, the method may determine whether the state of the rear oxygen sensor 15 satisfies the diagnosis entry condition, while step S110 is repeated.

At step S120, the method may determine whether an output voltage of the rear oxygen sensor 15 is included in the preset fault determination voltage range. When the output voltage of the rear oxygen sensor 15 exists between the lowest limit and the highest limit of the fault determination voltage range, the method may determine that the rear oxygen sensor 15 has a fault. When it is determined at step S120 that the output voltage of the rear oxygen sensor 15 is included in the fault determination voltage range, the method may determine that the rear oxygen sensor 15 is opened.

However, although the output voltage of the rear oxygen sensor 15 is included in the fault determination voltage range R, that is, between the lowest limit and the highest limit of the fault determination voltage range R, the output voltage may be temporarily included in the fault determination voltage range, due to various factors such as disturbance. Thus, steps S130 and S140 which will be described below may be performed in order not to confirm that the rear oxygen sensor 15 is opened, even though the output voltage of the rear oxygen sensor 15 is included in the fault determination voltage range.

Thus, although it is determined at step S120 that the output voltage of the rear oxygen sensor 15 is included in the fault determination voltage range, the method may not confirm that the rear oxygen sensor 15 is opened.

At step S130, the method may increase a diagnosis count by 1, when it is determined at step S120 that the output voltage of the rear oxygen sensor 15 is included in the fault determination voltage range. Specifically, when it is determined at step S120 that the output voltage of the rear oxygen sensor 15 is included in the fault determination voltage range, the method may temporarily determine that the rear oxygen sensor 15 has a fault or is opened, and record the fault by increasing the diagnosis count.

At step S140, the method may compare the fault diagnosis count, which was newly increased at step S130, to a preset heater duty change count. At step S130, a fault diagnosis count may be increased whenever it is determined at step S120 that the output voltage of the rear oxygen sensor 15 falls within the fault determination voltage range, and then compared to the heater duty change count.

As described above, although it is determined at step S120 that the output voltage of the rear oxygen sensor 15 falls within the fault determination voltage range, the method may not confirm that the rear oxygen sensor 15 has a fault, but determine whether to confirm that the rear oxygen sensor 15 has a fault, through step S140.

At step S150, the method may change power applied to the heater 15 c which is installed at one side of the rear oxygen sensor 15 so as to activate the rear oxygen sensor 15, in order to change the output voltage of the rear oxygen sensor 15. The heater 15 c may receive power to maintain the activated state of the rear oxygen sensor 15, and the power may be applied to the heater at a preset duty. For example, the duty of the power applied to the heater may be set to a value close to almost 100%.

At step S150, the method may change the duty of the power applied to the heater 15 c for a preset time. For example, the method may change the duty to 0% for a preset time of 5 seconds, and cut off the power applied to the heater 15 c. When the duty of the heater is changed, the voltage of the rear oxygen sensor 15 may be changed. Using such a characteristic, the method may determine whether the rear oxygen sensor 15 is opened. When the rear oxygen sensor 15 is normal and the duty of the heater 15 c is 0% or the power of the heater 15 c is cut off, the voltage of the rear oxygen sensor 15 may be increased. As such, the method may determine whether the rear oxygen sensor 15 is opened, using the principle that the voltage of the rear oxygen sensor 15 is changed according to the change of the heater duty.

The time during which the duty of the power applied to the heater is changed may be set to the time during which the activation temperature of the rear oxygen sensor 15 can be maintained. That is, although the power applied to the heater is cut off, the heater may maintain the activation temperature.

At step S160, the method may monitor the output voltage of the rear oxygen sensor 15 after step S150 is performed. At step S160, the method may determine whether the output voltage of the rear oxygen sensor 15 falls within the fault determination voltage range.

At step S160, the method may determine whether the output voltage of the rear oxygen sensor 15 falls within the fault determination voltage range R, or exists between the lowest limit and the highest limit of the fault determination voltage range, as in step S120.

Since step S160 is performed after step S150 unlike step S120, step S160 may be performed to confirm whether the rear oxygen sensor 15 has a fault.

The fault determination voltage ranges of steps S120 and S160 may be set to the same range.

At step S170, the method may confirm that the rear oxygen sensor 15 has a fault or is opened, when it is determined at step S160 that the output voltage of the rear oxygen sensor 15 is included in the fault determination voltage range.

When it is determined at step S160 that the output voltage of the rear oxygen sensor 15 is included in the fault determination voltage range R, the procedure may return to step S110. When the procedure returns to step S110 from step S160, the method may pass through step S180 to reset the fault diagnosis count.

In accordance with the embodiment of the present disclosure, when the voltage of the rear oxygen sensor 15 is included in the fault determination voltage range R as illustrated in FIG. 6, the method for detecting a fault of a rear oxygen sensor may determine whether the fault diagnosis count reaches a predetermined value, while increasing the fault diagnosis count. When the fault diagnosis count reaches the preset value, the method may change the duty of the heater 15 c so as to temporarily cut off the power of the heater 15 c, and monitor the voltage behavior. Then, when the voltage of the rear oxygen sensor 15 is normally changed according to the duty change of the heater 15 c, the method may determine that the rear oxygen sensor 15 is not opened, and reset the counter.

FIG. 7 illustrates a system for detecting a fault of a rear oxygen sensor in accordance with an embodiment of the present disclosure.

The system for detecting a fault of a rear oxygen sensor in accordance with the embodiment of the present disclosure may include a voltage comparator 21, a counter 22, a heater power controller 23, and a controller 24, and perform the above-described method for detecting a fault of the rear oxygen sensor. The voltage comparator 21 may determine whether an output voltage of the rear oxygen sensor 15 falls within a preset fault determination voltage range R. The counter 22 may increase a fault diagnosis count when the output voltage of the rear oxygen sensor 15 falls within the fault determination voltage range, and determine whether the fault diagnosis count reaches a previously stored heater duty change count. The heater power controller 23 may decrease the duty of the power applied to the heater 15 c installed at the rear oxygen sensor 15 when the fault diagnosis count reaches the previously stored heater duty change count. The controller 24 may (i) determine whether the rear oxygen sensor 15 satisfies a condition for diagnosing a fault of the rear oxygen sensor 15, based on information inputted from an electronic control unit (ECU) of the vehicle, (ii) control the counter 22 to increase the fault diagnosis count when the voltage comparator 21 determines that the output voltage of the rear oxygen sensor 15 is included in the fault determination voltage range R, and (iii) control the heater power controller 23 to decrease the duty of the heater, when the fault diagnosis count stored in the counter 22 reaches the previously stored heater duty change count.

The voltage comparator 21 may determine whether the output voltage of the rear oxygen sensor 15 is included in the fault determination voltage range R. The voltage comparator 21 may determine whether the output voltage of the rear oxygen sensor 15 is larger than the lowest limit of the fault determination voltage range R and smaller than the highest limit of the fault determination voltage range R.

When the voltage comparator 21 determines that the output voltage of the rear oxygen sensor 15 is included in the fault determination voltage range R, the counter 22 may increase the fault diagnosis count stored therein by 1. Furthermore, the counter 22 may compare the fault diagnosis count to the heater duty change count stored therein. That is, the counter 22 may determine whether the fault diagnosis count reaches the heater duty change count, while increasing the fault diagnosis count by 1.

The heater power controller 23 may control power applied to the heater 15 c. The heater power controller 23 may turn on/off the power supplied to the heater 15 c, and control the heater 15 c to heat and activate the rear oxygen sensor 15, while controlled by the controller 24 or the ECU. The heater power controller 23 may control the power applied to the heater 15 c such that the duty of the power is decreased. Desirably, the heater power controller 23 may control the power applied to the heater 15 c such that the duty of the power is decreased to 0%. Furthermore, the duty of the power applied to the heater 15 c may be changed or decreased only for a predetermined time in which the rear oxygen sensor 15 maintains the activation temperature.

Based on the information inputted from the ECU, the controller 24 may determine whether the state of the rear oxygen sensor 15 satisfies the condition for diagnosing a fault of the rear oxygen sensor 15. When the fault diagnosis condition is satisfied, the controller 24 may control the voltage comparator 21, the counter 22, and the heater power controller 23. The controller 24 may control the counter 22 to increase the fault diagnosis count stored in the counter 22, when the voltage comparator 21 determines that the output voltage of the rear oxygen sensor 15 is included in the fault determination voltage range R, and control the heater power controller 23 to control the duty of the power applied to the heater 15 c, when the fault diagnosis count reaches the preset heater duty change count.

The system for detecting a fault of the rear oxygen sensor in accordance with the embodiment of the present disclosure may be installed in the ECU, and perform the method for detecting a fault of the rear oxygen sensor.

In accordance with the embodiments of the present disclosure, when the output voltage of the rear oxygen sensor falls within the fault determination voltage range in a state where the brake pedal and the accelerator pedal are not operated while the vehicle is coasting on a long downhill, the method and system may temporarily cut off the power of the heater to heat the rear oxygen sensor, and observe the voltage behavior of the rear oxygen sensor. Then, only when the voltage of the rear oxygen sensor falls within the fault determination voltage range, the method and system may confirm that the rear oxygen sensor is opened.

Thus, the method and system may substantially prevent the case in which it is determined that the rear oxygen sensor is opened, according to a driver's driving habit. Therefore, it is possible to substantially prevent an error in which the rear oxygen sensor is replaced even when the rear oxygen sensor is not opened.

While the present disclosure has been described with respect to the specific embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the spirit and scope of the disclosure as defined in the following claims. 

What is claimed is:
 1. A method for detecting a fault of a rear oxygen sensor which is installed at the rear of a catalytic converter on an exhaust pipe of a vehicle and measures the concentration of oxygen included in exhaust gas passing through the catalytic converter, the method comprising: determining whether the rear oxygen sensor satisfies a diagnosis entry condition; comparing an output voltage of the rear oxygen sensor to a preset fault determination voltage range so as to determine whether the output voltage of the rear oxygen sensor is included in the fault determination voltage range; changing the duty of a heater of the rear oxygen sensor when the output voltage of the rear oxygen sensor is included in the fault determination voltage range; determining whether the output voltage of the rear oxygen sensor, controlled by changing the heater duty, is included in the fault determination voltage range; and confirming that the rear oxygen sensor has a fault, when the output voltage of the rear oxygen sensor is included in the fault determination voltage range.
 2. The method of claim 1, wherein in the determining whether the rear oxygen sensor satisfies the diagnosis entry condition, the diagnosis entry condition is satisfied when the temperature of the rear oxygen sensor exceeds a preset temperature.
 3. The method of claim 1, wherein in the determining whether the rear oxygen sensor satisfies the diagnosis entry condition, the diagnosis entry condition is satisfied when an engine is completely warmed up.
 4. The method of claim 1, wherein in the determining whether the rear oxygen sensor satisfies the diagnosis entry condition, the diagnosis entry condition is satisfied when control for an air/fuel ratio of an engine is performed.
 5. The method of claim 1, wherein in the determining whether the rear oxygen sensor satisfies the diagnosis entry condition, the diagnosis entry condition is satisfied when the vehicle is not coasting.
 6. The method of claim 1, wherein in the comparing the output voltage of the rear oxygen sensor to the preset fault determination voltage range so as to determine whether the output voltage of the rear oxygen sensor is included in the fault determination voltage range, the procedure returns to the determining whether the rear oxygen sensor satisfies the diagnosis entry condition when the output voltage of the rear oxygen sensor is included in the fault determination voltage range.
 7. The method of claim 1, wherein in the changing the duty of the heater of the rear oxygen sensor, the duty of the heater is changed to minimum from maximum when the output voltage of the rear oxygen sensor is included in the fault determination voltage range.
 8. The method of claim 1, wherein the changing the duty of the heater of the rear oxygen sensor is performed while the rear oxygen sensor maintains an activation temperature.
 9. The method of claim 8, wherein in the changing the duty of the heater of the rear oxygen sensor, the duty of the heater is changed for 4 seconds to 6 seconds when the output voltage of the rear oxygen sensor is included in the fault determination voltage range.
 10. The method of claim 1, further comprising, between the steps of comparing the output voltage of the rear oxygen and the changing the duty of the heater of the rear oxygen sensor when the output voltage of the rear oxygen sensor is included in the fault determination voltage range: determining that the rear oxygen sensor has a fault, when the output voltage of the rear oxygen sensor is included in the fault determination voltage range, and increasing a fault diagnosis count; and comparing the fault diagnosis count to a preset heater duty change count so as to determine whether the fault diagnosis count is larger than the heater duty change count.
 11. The method of claim 10, wherein in the determining whether the output voltage of the rear oxygen sensor, controlled by changing the heater duty, is included in the fault determination voltage range, when the output voltage of the rear oxygen sensor is not included in the fault determination voltage range, the fault diagnosis count is reset, and the procedure returns to the determining whether the rear oxygen sensor satisfies the diagnosis entry condition.
 12. The method of claim 1, wherein in the comparing the output voltage of the rear oxygen sensor to the preset fault determination voltage range so as to determine whether the output voltage of the rear oxygen sensor is included in the fault determination voltage range and the determining whether the output voltage of the rear oxygen sensor, controlled by changing the heater duty, is included in the fault determination voltage range, the fault determination voltage ranges are set to the same range.
 13. A system for detecting a fault of a rear oxygen sensor, comprising: a voltage comparator configured to determine whether an output voltage of a rear oxygen sensor is included in a preset fault determination voltage range; a counter configured to increase a fault diagnosis count, when the voltage comparator determines that the output voltage of the rear oxygen sensor is included in the fault determination voltage range, and determine whether the fault diagnosis count reaches a previously stored heater duty change count; a heater power controller configured to control power applied to a heater installed at the rear oxygen sensor such that the duty of the power is decreased, when the fault diagnosis count reaches the heater duty change count; and a controller configured to (i) determine whether a condition for diagnosing a fault of the rear oxygen sensor is satisfied, based on information inputted from an electronic control unit (ECU) of a vehicle, (ii) control the counter to increase the fault diagnosis count when the voltage comparator determines that the output voltage of the rear oxygen sensor is included in the fault determination voltage range, and (iii) operate the heater power controller to decrease the duty of the heater, when the fault diagnosis count stored in the counter reaches the preset heater duty change count.
 14. The system of claim 13, wherein the heater power controller decreases the duty of the power applied to the heater to minimum for a preset time.
 15. The system of claim 13, wherein the heater power controller decreases the duty of the power applied to the heater for a time in which the rear oxygen sensor maintains an activation temperature. 